1. Field of the Invention
This invention relates to two-stage processes for purifying wastewater containing organic and adsorbable pollutants and, more particularly, to such processes including a biophysical treatment first stage and an adsorbent contact treatment second stage.
2. Related Prior Art
The problem of water pollution is widely recognized and has resulted in local, state and federal regulations. In response to these regulations, as well as in the Public interest, various treatment methods have been developed to remove the pollutants found in wastewater.
The quantity of pollutants in wastewater is commonly determined by measuring the amount of dissolved oxygen required to biologically decompose the waste organic matter in polluted water. This measurement, called biochemical oxygen demand (BOD), provides an index of the organic pollution of water. Some organic contaminants, such as chlorinated aromatics, are not amenable to conventional biological decomposition and tests such as chemical oxygen demand (COD) and total organic carbon (TOC) have been employed to measure the concentration of these compounds.
A particularly useful process for removing pollutants from wastewater employs a mixture of bacteria and powdered activated carbon in a treatment zone. This process, called the PACT.TM. treatment system, is disclosed in Hutton et al., U.S. Pat. Nos. 3,904,518 and 4,069,148. The PACT treatment system operates as a continuous flow process with an aeration basin followed by a separate clarifier to separate biologically active solids and carbon from the treated wastewater and the settled sludge is returned to the aeration basin.
A slightly different biophysical treatment process is described by McShane et al., in "Biophysical Treatment of Landfill Leachate Containing Organic Compounds" Proceedings of Industrial Waste Conference, 1986 (Pub. 1987), 41st, 167-77. In this process a biological batch reactor is used with powdered activated carbon and the system is operated in the "fill and draw" mode, also known as the sequenced batch reactor (SBR) mode. A similar scheme for treatment of leachate is disclosed in Ying et al., U.S. Pat. No. 4,623,464 in which a SBR is operated with both biologically active solids and carbon present to treat PCB and a dioxin-containing leachate.
Another single-vessel method of biological wastewater treatment is the Intermittent Cycle Extended Aeration System (ICEAS) described by Goronszy in Journal Water Pollution Control Federation, Vol. 51, No. 2, February, 1979, pp. 274-287.
Brown et al., U.S. Pat. No. 4,468,327 discloses a singl vessel biological treatment process, known as extended aeration, in which a single vessel fitted with at least one inlet baffle continuously receives influent and intermittently aerates, settles and decants biologically treated effluent.
Nicol, U.S. Pat. No. 3,524,547 discloses a sewage treatment plant which includes an inlet compartment and two treatment compartments. Sewage from the inlet compartment is transferred substantially continuously from the inlet compartment into a first treatment compartment, from the first treatment compartment to a second treatment compartment and treated sewage is removed from the second treatment compartment. The flow is then reversed, flow being from the inlet compartment to the second treatment compartment, to the first compartment and finally from the system. Thus, the two treatment compartments alternately serve as the first and second treatment stages.
Goronszy, U.S. Pat. No. 4,663,044 discloses a continuous inflow, activated sludge treatment process employing three zones for biological adsorption and biological degradation. Activated sludge contacts wastewater in a separate first zone to adsorb biodegradable dissolved compounds. The sludge wastewater mixture passes to interconnected second and third zones where aeration and settling are followed by decanting of biologically treated effluent.
These different straight biological treatment methods often do not produce a treated wastewater of suitable quality for discharge to the environment. Likewise, the use of a mixture of biologically active solids and powdered activated carbon in the same system may be incapable of Producing adequate treatment to meet discharge requirements for wastewaters containing compounds which are difficult to biodegrade and only weakly adsorbed on carbon.